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| United States Patent |
5,625,011
|
|
Palsule
|
April 29, 1997
|
Molecular composites, process for the preparation of said composites and
their uses
Abstract
The invention relates to molecular composites including a rigid reinforcing
thermoplastic polymer dispersed on a molecular scale in a flexible
organosiloxane/thermoplastic polymer copolymer, to a process for the
preparation of the said composites and to a material with a surface
enriched in siloxane, obtained from the said molecular siloxane
composites, and to the uses of said composites and of the said material,
in particular for aerospace construction.
| Inventors:
|
Palsule; Sanjay (Juniper Green, GB6)
|
| Assignee:
|
Agence Spatiale Europeenne (Paris, FR)
|
| Appl. No.:
|
267583 |
| Filed:
|
June 29, 1994 |
Foreign Application Priority Data
| Current U.S. Class: |
525/431; 525/421; 525/425; 525/433 |
| Intern'l Class: |
C08L 079/08 |
| Field of Search: |
525/431,421,425,437
|
References Cited
U.S. Patent Documents
| 4387193 | Jun., 1983 | Giles, Jr. | 525/431.
|
| 4631318 | Dec., 1986 | Hwang et al. | 525/432.
|
| 4735999 | Apr., 1988 | Patterson et al. | 525/431.
|
| 4816527 | Mar., 1989 | Rock | 525/431.
|
| 4977223 | Dec., 1990 | Arnold et al. | 525/432.
|
| 5387639 | Feb., 1995 | Sybert et al. | 524/537.
|
| Foreign Patent Documents |
| 0329956 | Aug., 1989 | EP.
| |
| 0417778 | Mar., 1991 | EP.
| |
Primary Examiner: Dean; Ralph H.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
I claim:
1. A molecular composite, comprising a rigid reinforcing thermoplastic
polymer dispersed on a molecular scale in a flexible
organosiloxane/thermoplastic polymer copolymer, wherein said rigid
reinforcing thermoplastic polymer is a polyamideimide.
2. The molecular composite according to claim 1, characterized in that the
surface of the said molecular composite is enriched in siloxane.
3. The molecular composite according to claim 1, wherein the flexible
organosiloxane/thermoplastic polymer copolymer is selected from the group
consisting of:
--polydimethylsiloxane/polyetherimide,
--polydimethylsiloxane/polycarbonate,
--polydimethylsiloxane/polysulphone,
--polydimethylsiloxane/polyamideimide,
--polydimethylsiloxane/polyester,
--polydimethylsiloxane/polyesteramide,
--polydimethylsiloxane/polyaryl ether,
--polydimethylsiloxane/polystyrene, and
--polydimethylsiloxane/polymethyl methacrylate.
4. The molecular composite according to claim 1, characterized in that the
flexible organosiloxane/thermoplastic polymer copolymer is a
polydimethylsiloxane/polyetherimide block copolymer.
5. The molecular composite according to claim 1, characterized in that it
contains between 1 and 98% by weight of the flexible
organosiloxane/thermoplastic polymer copolymer.
6. A process for the preparation of a molecular composite wherein a rigid
reinforcing thermoplastic polymer is dispersed on a molecular scale in a
flexible organosiloxane/thermoplastic polymer copolymer comprising the
following steps of:
a) dissolving an appropriate quantity of the rigid thermoplastic polymer in
a suitable solvent and separately dissolving an appropriate quantity of
the flexible organosiloxane/thermoplastic polymer copolymer in the same
solvent, until completely dissolved, wherein said rigid reinforcing
thermoplastic polymer is a polyamideimide;
b) homogeneously mixing the solutions obtained in step a);
c) adding the solution obtained at the end of step b) to a large volume of
water, which causes the formation of flecks of molecular siloxane
composite in a suspension in water;
d) recovering said flecks of molecular siloxane composite by washing and
drying the same.
7. The process according to claim 6, characterized in that the drying of
the flecks includes a step of drying in a vacuum oven at a temperature of
approximately 70.degree. C. for 12 to 48 hours, then at a temperature of
approximately 150.degree. C. for 12 to 48 hours and, finally, at a
temperature approximately 20.degree. C. higher than the glass transition
temperature of the rigid reinforcing thermoplastic polymer for 12 to 48
hours.
8. A material whose surface is enriched in siloxane, wherein the material
is obtained by moulding a molecular composite comprising a rigid
reinforcing thermoplastic polymer dispersed on a molecular scale in a
flexible organosiloxane/thermoplastic polymer copolymer, wherein said
rigid reinforcing thermoplastic polymer is a polyamideimide.
9. The material according to claim 8, characterized in that the moulding of
the molecular composite is performed under pressure at a temperature
30.degree. C. higher than the glass transition temperature of the rigid
thermoplastic polymer.
10. The material according to claim 8, wherein the material is an isotropic
material.
11. The material according to claim 8, wherein the material is an
anisotropic material.
12. A process for the preparation of a molecular composite wherein a rigid
reinforcing thermoplastic polymer is dispersed on a molecular scale in a
flexible organosiloxane/thermoplastic polymer copolymer, characterized in
that the composite is obtained by melt-blending, wherein said rigid
reinforcing thermoplastic polymer is a polyamideimide.
13. An aerospace construction comprising a molecular composite wherein a
rigid reinforcing thermoplastic polymer is dispersed on a molecular scale
in a flexible organosiloxane/thermoplastic polymer copolymer, wherein said
rigid reinforcing thermoplastic polymer is a polyamideimide.
14. An aerospace construction comprising the material according to claim 8.
Description
The present invention relates to a new class of siloxane derivatives which
can be employed in the aerospace industry.
Polymers conventionally employed in the aerospace industry, such as
polyimides, polyesters and carbon/epoxy composites, have the disadvantage
of being rapidly degraded under the thermooxidative attack by atomic
oxygen in a low Earth orbit.
It is known that polysiloxanes are stable in the environment of the low
Earth orbit. A review of the effects of atomic oxygen on polysiloxanes and
on other polymers has been published, for example, by Dauphin ["Atomic
oxygen: A Low orbit Plague", in: Looking Ahead for Materials and
Processes, Material Science Monographs, 41 (Eds. J. de Bossu, G. Briens,
and P. Lissacs), Elsevier Science Publishers, Amsterdam, pp. 345-367,
(1987)]. However, polysiloxanes have mediocre thermal and mechanical
properties and a low glass transition temperature, which result from the
low energy of cohesion between the polymer chains of which they consist,
and make them useless as structural materials for space vehicles.
There are also known copolymers resulting from the copolymerization of
siloxane monomers with monomers which usually form part of the composition
of thermoplastic polymers; these copolymers will be referred to as
"organosiloxane/thermoplastic polymer copolymers" in the description which
follows.
Just like polysiloxanes, organosiloxane/thermoplastic polymer copolymers do
not have a glass transition temperature or mechanical properties that are
compatible with use as structural materials for space vehicles.
Some mixtures of flexible organosiloxane/thermoplastic polymer copolymers
with thermoplastic polymers have been described as having high glass
transition temperatures. However, they are not considered to have the
mechanical properties needed in order to be employed as structural
materials for space vehicles. For example, Arnold et al., [High
Performance Polymers, vol. 2, p. 83, (1990] have developed a mixture of
polybenzimidazole and of polysiloxaneimide block copolymer, at the surface
of which the siloxane predominates. Studies in an environment simulating
the low Earth orbit conditions, as well as the study of the action of
atomic oxygen on these mixtures, have shown that the siloxane present at
the surface is converted into inorganic silicate in the presence of atomic
oxygen. The glass transition temperatures of these mixture are high, but
no indication concerning their mechanical properties is given.
Patel et al., [Macromolecules, vol. 21, pp. 2689-2696, (1988)] have
developed a mixture of polysulphone/copolymerized polysiloxane and
polysulphone in which the enrichment of the surface in siloxane was
relatively high. On the other hand, no result concerning the reinforcement
on a molecular level of the siloxane copolymer in the mixture is provided,
and no study relating to the thermal and mechanical properties of the
mixture has been carried out.
Injection moldable blends of silicone copolymer and polyetherimide (which
is not a rigid thermoplastic polymer are described in EP 0 329 956 and
U.S. Pat. No. 4,387,193. These materials are not reinforced at molecular
level, and need the addition of reinforcing material to increase their
mechanical properties.
Furthermore, materials consisting of a polymer with a flexible helical
structure and reinforced on a molecular level by a rodlike rigid polymer
are known by the name of "molecular composites". For a review of the main
molecular composites and of their properties see, for example, Pawlikoski
et al.: [Ann. Rev. Mater. Sci. vol. 21, pp. 159-184, (1991)]. Molecular
composites are prepared by mixing their consonants in solution, at
concentration lower than the critical concentration point defined for the
ternary system:rigid polymer/flexible copolymer/solvent, and rapidly
coagulating the mixture.
U.S. Pat. No. 4,631,318 describes a molecular composite as a material
consisting of a rigid reinforcing polymer dispersed on a molecular level
in a thermoplastic polymer with a flexible helical structure. This
molecular composite is analogous to a fibre-reinforced composite.
Hwang et al. [J. Macromol. Sci. Phys., vol. B 22(2), pp. 231-257, (1983)]
have developed a molecular composite consisting of
poly-p-phenylenebenzobisthiazole and of poly-2,5(6)benzimidazole, which
has excellent tensile properties.
Takayanagi et al. [J. Macromol. Sci. Phys., vol. B 17(4), p. 591, (1980)]
have developed molecular-level composites by dispersing rigid;
microfibrils of rigid aromatic polyamides in flexible aliphatic
polyamides.
Despite the known thermal and mechanical properties of molecular
composites, no study has made it possible to end in the development of
molecular composites which have specific surface characteristics capable
of enabling the material to be stable in the low Earth orbit environment
in the presence of molecular oxygen. In particular, no molecular composite
having a surface preferentially enriched in one of its components has been
known until now.
However, according to the present Invention, it has been now discovered
that a flexible organosiloxane polymer can be reinforced at a molecular
level with a rigid thermoplastic polymer, and this makes it possible to
obtain a new class of oxidation-resistant molecular composites called
"molecular siloxane composites" hereinafter, in which the thermal and
mechanical molecular-level reinforcement of the siloxane copolymer by the
rigid thermoplastic polymer by the rigid thermoplastic polymer is
accompanied by a siloxane-enriched surface which stabilizes the material
in the low Earth orbit environment and allows it to withstand the attack
by atomic oxygen.
The subject of the present invention is a molecular composite called
molecular siloxane composite, characterized in that it includes a rigid
reinforcing thermoplastic polymer dispersed on a molecular scale in a
flexible organosiloxane/thermoplastic polymer copolymer.
The molecular siloxane composites in accordance with the invention are
analogous on a molecular scale to fibre-reinforced quasiisotropic
composites.
The flexible organosiloxane/thermoplastic polymer copolymer and the rigid
thermoplastic polymer, which form a molecular siloxane composite in
accordance with the invention, are thermodynamically miscible. A molecular
siloxane composite in accordance with the invention therefore has a single
glass transition point which is intermediate between that of the two
polymers of which it consists.
According to a preferred embodiment of the present invention the surface of
the said molecular siloxane composite is enriched in siloxane.
When a molecular composite in accordance with the invention is placed in an
environment such as that of the low Earth orbit the siloxane which
predominates at the surface of the said composite reacts with atomic
oxygen to form a protective layer of silica at the surface.
The molecular siloxane composites in accordance with the invention have
thermal and mechanical properties which are superior to those of the
organosiloxane/thermoplastic polymer copolymers and of the mixtures of
organosiloxane/thermoplastic polymer copolymers with flexible
thermoplastic polymers known in the prior art, which proves the existence
of a molecular-scale reinforcement of the flexible
organosiloxane/thermoplastic polymer copolymer by the rigid thermoplastic
polymer.
The molecular siloxane composites in accordance with the invention
therefore constitute a new material which can be employed in particular as
structural material for aerospace. In fact, by virtue of the thermal and
mechanical properties and of the surface properties of the molecular
siloxane composites in accordance with the invention, in a low Earth orbit
this material maintains its dimensional stability and its thermomechanical
properties, stands up to the space environment (that is to say in
particular to ultraviolet radiation and to space debris) and additionally
maintains these characteristics despite the cyclic thermal changes which
take place in a low Earth orbit.
All polysiloxane copolymers with a thermoplastic polymer of the linear,
grafted or block type can be employed to form the "flexible
organosiloxane/thermoplastic polymer copolymer" component of the molecular
siloxane composite in accordance with the invention. As nonlimiting
examples of such copolymers there will be mentioned:
--polydimethylsiloxane/polyetherimide,
--polydimethylsiloxane/polycarbonate,
--polydimethylsiloxane/polysulphone,
--polydimethylsiloxane/polyamideimide,
--polydimethylsiloxane/polyester,
--polydimethylsiloxane/polyesteramide,
--polydimethylsiloxane/polyaryl ether,
--polydimethylsiloxane/polystyrene,
--polydimethylsiloxane/polymethyl methacrylate, and the like.
All thermoplastic polymers in which the chains are rigid because of a high
rotational energy barrier of the structural units of which they consist
can be employed as rigid reinforcing polymers for implementing the present
invention. Such polymers are, for example and without any limitation being
implied, poly-p-phenylene terephthalamide, polyamideimide,
poly-p-benzamide, poly-p-phenyleneoxydiphenyleneterephthalamide,
poly-p-phenylenebenzobisthiazole, poly-p-phenylenebenzobisoxazole,
poly-p-phenylenes containing a linear chain, and the like.
The molecular siloxane composites in accordance with the invention may
include any proportion of the rigid polymer and of the flexible
organosiloxane/thermoplastic polymer copolymer; for example, they may
include between 1 and 98% by weight of the flexible
organosiloxane/thermoplastic polymer copolymer.
Another subject of the present invention is a process for the preparation
of the molecular siloxane composites as defined above.
This process is characterized in that it includes the following stages:
a) a step during which an appropriate quantity of the rigid thermoplastic
polymer is dissolved in a suitable solvent and an appropriate quantity of
the flexible organosiloxane/thermoplastic polymer copolymer is dissolved
separately, in the same solvent as that employed for dissolving the rigid
thermoplastic polymer, until completely dissolved;
b) a step during which the solutions obtained in step a) are mixed
homogeneously;
c) a step during which the solution obtained at the end of stage b) is
added to at least 20 times its volume of water, which causes the formation
of flocs of molecular siloxane composite in suspension in water;
d) steps of recovery, of washing and of drying of the said flakes of
molecular siloxane composite.
According to a preferred embodiment of the process in accordance with the
present invention the solution obtained at the end of step b) is added to
approximately 50 times its volume of water.
According to another preferred method of implementing the process in
accordance with the invention the drying of the said flakes includes a
step of drying in a vacuum oven at a temperature of approximately
70.degree. C. for 12 to 48 hours, then at a temperature of approximately
150.degree. C. for 12 to 48 hours and, finally, at a temperature
approximately 20.degree. C. higher than the glass transition temperature
of the rigid thermoplastic polymer for 12 to 48 hours.
To guide the choice of a solvent making it possible to implement the
process in accordance with the invention, it will be noted that a suitable
solvent must make it possible to dissolve the rigid thermoplastic polymer
as well as the flexible organosiloxane/thermoplastic polymer copolymer
without degrading the structure of the polymer chains. The choice of this
solvent is therefore related to the chemical structures of both polymers.
Insofar as the solvent for the organosiloxane compolymer is concerned, the
solvent which dissolves the organic component of the said copolymer must
be chosen.
Solvents which are suitable for most polymers are known; reference may be
made, for example, to the list published by Fuchs in the Polymer Handbook
[Brandrup & Immergut (Eds.); John Wiley, N.Y., (1990)].
In the case of a polymer for which no description of the desirable solvent
is available, a person skilled in the art can easily determine whether a
particular solvent is suitable by carrying out simple tests; for example
attempts may be made to dissolve separately the rigid polymer and the
flexible organosiloxane/thermoplastic polymer copolymer in a quantity of
10% by weight in the solvent to be tested and a check may be made, after
stirring for 12 to 36 hours, whether the dissolution is complete or
incomplete.
On the basis of the literature data and of the tests indicated above, it
has been found, for example, that the polyamideimide (in the form of its
polyamic acid precursor) and poly-p-phenyleneoxydiphenyleneterephthalamide
can be dissolved by dimethylacetamide or N-methylpyrrolidinone, in the
presence of 1 or 2% of calcium chloride (molar percentage). Other rigid
polymers such as poly-p-phenyleneterephthalamide and polybenzobisthiazole
can be dissolved using sulphuric acid.
Flexible organosiloxane copolymers such as
polydimethylsiloxane/polyetherimide or polydimethylsiloxane/polyamideimide
can be dissolved using dimethylacetamide or using N-methylpyrrolidinone.
Unexpectedly, and in contrast with the processes of preparation of
molecular composites of the prior art, the process of the Invention does
not necessitate to define a critical concentration point and to operate at
a concentration lower than said critical concentration point.
A process for the preparation of siloxane composites in accordance with the
invention preferably includes the following steps:
1) dissolving the rigid polymer (whose glass transition temperature has
been determined beforehand) in a suitable solvent, to obtain a solution
containing approximately 10% by weight of the said polymer;
2) dissolving the flexible organosiloxane/thermoplastic polymer copolymer
(whose glass transition temperature has been determined beforehand) in the
same solvent as that employed for step 1, in order to obtain a solution
containing approximately 10% by weight of the said polymer;
3) mixing the solutions obtained in stage 1 and in step 2, with stirring,
to obtain a homogeneous solution;
4) coagulating the solution obtained at the end of step 3 in a large volume
of water, in order to obtain flakes of molecular siloxane composite in
suspension in water;
5) filtering the flakes/water mixture to collect the flake of molecular
siloxane composite;
6) carefully washing the flakes with running water to remove all solvent
residues;
7) washing the flakes obtained at the end of stage 6 in acetone to remove
the aqueous wash residues;
8) drying the flakes washed with acetone in an air oven to remove a maximum
quantity of residual solvent and of the wash liquids;
9) drying the flakes obtained at the end of step 8 in a vacuum oven at a
temperature of approximately 70.degree. C. for two days, then at a
temperature of approximately 150.degree. C. for two days and, finally, at
a temperature approximately 20.degree. C. higher than the glass transition
temperature of the rigid polymer for two days.
The flakes of molecular siloxane composite in accordance with the invention
can then be converted, by any technique known per se, for example may be
moulded to the desired shape, and make it possible to obtain a material
the surface of which is enriched in siloxane.
Also in accordance with the invention, since no critical concentration
point is to be taken in account in its process of obtention, a molecular
siloxane composite in accordance witch the invention can be prepared by
the melt-blending or injection-moulding technique.
For instance, polyamide-imide and polysiloxane-imide can be blended, and
the mixture extruded at a temperature above 303.degree. C.
According to a preferred embodiment the material based on molecular
siloxane composite in accordance with the present invention is an
isotropic material.
According to another preferred embodiment the material based on molecular
siloxane composite in accordance with the present invention is an
anisotropic material.
The formation of an isotropic material or of an anisotropic material is
determined by the proportion of rigid thermoplastic polymer. In fact, if
the percentage of rigid thermoplastic polymer exceeds a certain threshold,
a preferential alignment of the chains of the said polymer is observed and
this imparts an anisotropic structure to the molecular composite.
The threshold from which an anisotropic material is obtained can vary
depending on the rigid polymer/flexible copolymer system employed.
However, a person skilled in the art can easily determine the appropriate
percentage of rigid thermoplastic polymer by carrying out a few tests and
by verifying the isotropic or anisotropic nature of the molecular siloxane
composite obtained, using a scanning electron microscope.
Pressure moulding of the flocs is advantageously undertaken at a
temperature approximately 30.degree. C. higher than the glass transition
temperature of the rigid thermoplastic polymer. For example, a film of
molecular siloxane composite in accordance with the invention can be
obtained in this way.
The present invention will be better understood with the aid of the
additional description which is to follow, which refers to examples of
preparation of molecular siloxane composites in accordance with the
invention.
It should be clearly understood, however, that these examples are given
solely by way of illustration of the subject of the invention and do not
constitute any limitation whatsoever thereof.
EXAMPLE 1
The rigid polymer employed is the polyamideimide (PAI) marketed under the
name of Torlon 4203 by the company Amoco Performance Products Inc. The
flexible organosiloxane/thermoplastic polymer copolymer employed is a
polysiloxane-imide (PSI) copolymer containing 37% by weight of siloxane
and marketed under the name of Siltem 1500 by the company G E Plastics.
15 g of polyamideimide in the form of its polyamic acid precursor were
dissolved in 150 ml of dimethylacetamide at a temperature of 60.degree. C.
85 g of PSI were separately dissolved in 850 ml of dimethylacetamide at
60.degree. C. The two solutions were mixed with stirring for 48 hours in
order to obtain a homogeneous solution.
This solution was then added to a volume of 5 liters of water, and this
causes the precipitation of the molecular siloxane composite in the form
of flocs in suspension in water. The flakes are next recovered by
filtration and then carefully washed with running water to remove all
solvent residues, and then with acetone to remove the aqueous wash
residues.
The flakes are next dried in an air oven and then in a vacuum oven at a
temperature of approximately 70.degree. C. for two days and then at a
temperature of approximately 150.degree. C. for two days and finally at a
temperature of 313.degree. C. for two days.
At the end of this heating the polyamic acid is converted into
polyamideimide with water elimination. Films of a thickness varying
between 0.1 and 1 mm are obtained by compression-moulding the flocs at a
temperature of 329.degree. C. The material obtained is isotropic. This
material was called ST-8515.
EXAMPLE 2
A molecular siloxane composite was prepared according to the protocol
described in Example 1 from 30 g of PAI dissolved in 300 ml of solvent and
from 70 g of PSI dissolved in 700 ml of solvent. The material obtained is
isotropic. This material was called ST-7030.
EXAMPLE 3
A molecular siloxane composite was prepared according to the protocol
described in Example 1 from 50 g of PAI dissolved in 500 ml of
dimethylacetamide and 50 g of PSI dissolved in 500 ml of
dimethylacetamide. This material was called ST 5050.
EXAMPLE 4
A molecular siloxane composite was prepared according to the protocol
described in Example 1 from 70 g of PAI dissolved in 700 ml of
dimethylacetamide and 30 g of PSI dissolved in 300 ml of
dimethylacetamide. This material was called ST 3070.
EXAMPLE 5
A molecular siloxane composite was prepared according to the protocol
described in Example 1 from 85 g of PAI dissolved in 850 ml of
dimethylacetamide and 10 g of PSI dissolved in 150 ml of
dimethylacetamide. This material was called ST 1585.
EXAMPLE 6
LABORATORY TESTS:
The degassing tests were performed in accordance with the protocol
described in Standard ESA-PSS-01-702.
The results of these tests are shown in Table I below.
TABLE I
______________________________________
MATERIAL TML.sup.a (%)
RML.sup.b VMC.sup.c (%)
______________________________________
ST-8515 0.30 0.08 0.01
ST-7030 0.66 0.08 0.01
______________________________________
.sup.a Total mass loss
.sup.b Recoverable mass loss
.sup.c Condensable volatile matter collected
The glass transition temperatures and the mechanical properties were also
determined and the results are assembled in Table II below.
TABLE II
______________________________________
ELONGA-
GTT.sup.a
STRENGTH MODULUS TION
MATERIAL (.degree.C.)
(MPa) (MPa) (%)
______________________________________
SI 172 18.90 23.89 40
ST-8515 187 21.25 98.96 38
ST-7030 199 -- -- --
ST-5050 222 -- -- --
ST-3070 279 31.56 374.6 13
ST-1585 283 -- -- --
______________________________________
.sup.a Glass transition temperature
TESTS IN REAL SPACE ENVIRONMENT:
The molecular siloxane composites obtained in Examples 1 and 2 were exposed
to the low Earth orbit environment during the ESA BIOPAN mission over a
period of 2 weeks. The sample was exposed for two weeks to an atomic
oxygen fluence of 8.66.times.10/20 atoms/cm.sup.2, to an ultraviolet
irradiation equivalent to 12 hours 30 min of solar irradiation and also to
cyclic thermal variations, as well as to the action of space debris and of
the space vacuum.
Table III shows the results of measurements performed by X-Ray photon
spectroscopy on the ST-8515 ST-7030 composites before and after exposure
to the low Earth orbit environment in the course of the BIOPAN mission.
TABLE III
______________________________________
ATOMIC
MATERIAL PEAK PERCENTAGE BE.sup.a
PMHW.sup.b
______________________________________
ST-8515 C 1s 60.0 287.0
2.34
unexposed O 1s 23.0 534 2.38
Si 2p 17.0 104.5
1.97
ST-8515 C 1s 42.8 289.0
2.25
exposed O 1s 33.6 536.6
2.65
Si 2p 23.6 107.5
2.67
ST-7030 C 1s 69.1 287.0
2.62
unexposed O 1s 16.9 534.8
2.10
Si 2p 14.0 104.1
2.45
ST-7030 C 1s 26.7 288.4
2.69
exposed O 1s 40.9 536.8
2.48
Si 2p 32.4 107.5
2.79
______________________________________
.sup.a bond energy
.sup.b peak midheight width
The X-Ray photons are emitted by an MgK source, at normal incidence
relative to the sample to be tested. The measurement is performed over a
depth of 50 .ANG.. It is found that in each of the samples the atomic
percentage of carbon has decreased after exposure, whereas those of
silicon and oxygen have increased. Furthermore, the peak mid-height width
(PMHW) increases for the 2 p peak of silicon and for the 1 s peak of
oxygen, whereas that of the 1 s peak of carbon remains practically
unaltered. This indicates an increase in the number of silicon-oxygen
bonds, which results from the conversion of the siloxane into a protective
silicone layer.
The increase in the oxygen content of the sample reflects the conversion of
the siloxane to silica and confirms the stability of the material in the
environmental conditions of the low Earth orbit. These results also
confirm that the material does not undergo any degradation due to the UV,
to space debris or to cyclic thermal variations.
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